1. Field of the Invention
This invention concerns a pH sensor and more specifically to a pH sensor that is inexpensive and high in performance.
2. Description of the Prior Art
The pH values of solid substances have been measured in order to examine its properties. For example, FIG. 7 is a diagram for explaining the method of measuring the pH value a rice grain for determination of the freshness of the rice grain.
In FIG. 7, reference numeral 21 is a rice grain, which is the object to be measured, 22 is water or other solvent in which a plurality of rice grains 21 are placed, and 23 is an ISFET (Ion sensitive field effect transistor) for pH measurement. That is, since ISFET 23 and other pH sensors are conventionally arranged for measurement of the pH of a liquid, when a solid is measured as the measured object, it was measured via water or other solvent 22 or a cloth or paper immersed in solvent 22.
Also, with a pH meter that uses an ordinary pH sensor 23, a standard solution, with a known pH and a pH buffer action, was used to perform calibration of the sensor so that the exact measuring value can be obtained.
However, the conventional procedure of placing and stirring a solid substance once in a solvent 22 and measuring the pH upon taking out solvent 22 not only required a large amount of the sample to be measured but much trouble was required as well. The measurement of the pH of a solid substance in a direct and simple manner was thus desired. However, the above-described prior-art pH sensor 23 cannot satisfy this demand.
Also as a device for making rapid measurements of solid substances, there is the pH distribution analysis device, with which a thin membrane of agar is formed on a detection plane and laser light is illuminated in a scanning manner with the measured object being placed on the thin membrane to measure the two-dimensional pH distribution. However, this device was unavoidably large and complex in structure. That is, this analysis device was expensive and did not enable measurements in a simple manner.
Furthermore, with a pH meter using a prior-art pH sensor, calibration work using a standard solution was required in all cases, and this has been a cause that prevented pH measurements from being made in a simple manner.